1. Field of the Invention
The present invention relates to an improved data transmission and monitoring system and method for transmitting and monitoring commands to devices including commands to open or close a valve or stop the valve somewhere in the middle of its travel. The present invention has been found to be particularly useful in the discrete state command and monitoring transmission art in industrial environments, especially as a device for controlling and monitoring motors and valves, and, hence, will be discussed with particular reference thereto. However, the present invention is applicable to many other types of discrete commands as well, as long as each operation on the device is of a discrete nature as opposed to continuous nature.
2. Description of the Prior Art
In the transmission and detection of commands to valves and motors, transmission systems are usually divided into two portions, one located where the commands are to originate, either by an automatic system or by a manual request from a human operator, and the other where the command is detected and routed to activate the controlled device and indicate the present state of the controlled device locally, as well as transmit the state back to the transmission means. Additional components are used to transmit the signal from the transmission portion to the receiving portion, including a power source to activate both the transmission and the receiving portion simultaneously and wires for carrying the signal. The system must be capable of transmitting signals to the remote location in such a manner that environmental factors which usually exist in industrial plants will not affect the signals transmitted. The system must also be reliable in operation for a long period of time and consistent in its manner of operation. In addition, the system must correctly perform a stop function to prevent actuation of the device.
Several types of transmission and detection systems have been known and used before, and typical examples thereof in the valve and motor command monitoring art are shown in U.S. Pat. No. 3,256,517, issued June 14, 1966, to T. Saltzberg et al.; U.S. Pat. No. 3,289,166, issued Nov. 29, 1966, to D. G. Emmel; U.S. Pat. No. 2,360,172, issued Oct. 10, 1944, to C. E. Stewart; U.S. Pat. No. 2,788,517, issued Apr. 9, 1957, to W. L. Smoot et al.; U.S. Pat. No. 3,251,992, issued May 17, 1966, to R. B. Haner, Jr.; U.S. Pat. No. 3,315,231, issued Apr. 18, 1968, to P. Belugou; U.S. Pat. No. 3,254,335, issued May 31, 1966, to R. F. Staten; U.S. Pat. No. 3,202,978, issued Aug. 24, 1965, to G. E. Lewis; U.S. Pat. No. 2,525,016, issued Oct. 10, 1950, to G. L. Borell; U.S. Pat. No. 2,003,047, issued May 28, 1935, to S. C. Henton et al.; U.S. Pat. No. 2,019,350, issued Oct. 29, 1935, to R. Koberich; U.S. Pat. No. 2,260,061, issued Oct. 21, 1941, to C. E. Stewart; and U.S. Pat. No. 2,992,366, issued July 11, 1961, to T. E. Veltfort, Jr.
The Saltzberg, Emmel and Stewart data transmission and collection systems use conventional coding techniques such as pulse coding or tone transmission to transmit information from the transmission device to the receiving device. However, this type of prior art requires complex logic for encoding and decoding data at the transmission device and at the receiving devices.
The Smoot, Haner, Belugou, Staten, Lewis and Borell devices use either direct current signals to transmit the information of three wires to transmit the information from the transmission device to the receiving device, requiring relatively high sustained voltage values which would be unsafe in an industrial environment or additional stringing of wires over long distances.
The Henton, Koberich, Stewart and Veltfort devices all use a different polarity current in a two-wire mode to transmit information from the transmission device to the receiving device but none of them disclose a stop function.
Another alternating polarity current transmission system is disclosed in FIG. 1 which has been used publicly and is part of the prior art. This system, however, requires additional relay contacts, as will be discussed in the Detailed Description of the Preferred Embodiment, to prevent the stop function from actuating the controlled device to move rather than to stop the controlled device.